Help! does anyone know what this growth is? I can't find photos of anything like it online, and this is my first betta fish. These photos are from a few days ago, and it's visibly growing each day-- it seems to sprout new clear bubbles on it making it larger every day. :( It is completely on the outside, but the innermost part of the growth is pinkish/orange like his scales, and then clear on the outside.

I'm guessing lymphocystis. Google that and see if that resembles what you are seeing.

Does he live alone? Lymphocystis is usually not fatal, it can be ugly, and it can sometimes hamper their ability to see or eat, depending on where the growth is. It can be spread to other fish (hence the questions about tank mates). I'm not sure what else to do, unless it breaks, in which case give him extra water cleanings to prevent infection.